High density lithium niobate photonic integrated circuits

Photonic integrated circuits have the potential to pervade into multiple applications traditionally limited to bulk optics. Of particular interest for new applications are ferroelectrics such as Lithium Niobate, which exhibit a large Pockels effect, but are difficult to process via dry etching. Here we demonstrate that diamond-like carbon (DLC) is a superior material for the manufacturing of photonic integrated circuits based on ferroelectrics, specifically LiNbO3. Using DLC as a hard mask, we demonstrate the fabrication of deeply etched, tightly confining, low loss waveguides with losses as low as 4 dB/m. In contrast to widely employed ridge waveguides, this approach benefits from a more than one order of magnitude higher area integration density while maintaining efficient electro-optical modulation, low loss, and offering a route for efficient optical fiber interfaces. As a proof of concept, we demonstrate a III-V/LiNbO3 based laser with sub-kHz intrinsic linewidth and tuning rate of 0.7 PHz/s with excellent linearity and CMOS-compatible driving voltage. We also demonstrated a MZM modulator with a 1.73 cm length and a halfwave voltage of 1.94 V.


LiNbO 3 etching with different methods
We compared the different dry etching methods for LiNbO 3 at first. With fluorine and chlorine chemistry plasma etching, we observe a rough surface due to the redeposition of non-volatile etch products LiF or LiCl (see Supplementary Fig. 2(a,b)). Hence, we selected the purely physical argon bombardment as the etching method. Using only vertical impingement plasma beams, we observe a slanted sidewall with a large number of redeposition products and distinctive trenching around the waveguide base (see Supplementary Fig. 2(c)). The observed trenching is related to the reflection of argon ions from the slanted sidewalls. By optimizing the angle between the argon ions and the wafer surface, we can reduce the redeposition of volatile etch products, and remove trenching by ion reflection (see Supplementary Fig. 2(d)), as well as increase the sidewall angle of the optical waveguide.

Electro-optic efficiency comparison for strip and ridge waveguides
We compare the electro-optic efficiency of two approaches to integrated lithium niobate photonics: First, the strip waveguide approach discussed in the manuscript and second, the ridge waveguide approach representative of state of the art in LiNbO 3 -based integrated photonics 1, 2 . We calculate the main metric of electro-optic modulation -the half-wave voltage length product, which describes the required voltage needed for the optical phase shift of π along a 1 cm electro-optic waveguides: is the effective electric field that takes into account the overlap integral between modulation electric field and the optical field distribution, r 33 is LiNbO 3 electrooptic coefficient, λ is optical wavelength, and n is the optical effective index 3 . We consider X-cut LiNbO 3 wafer and TE-polarization of optical mode, such that the principal axis of the optical and electric fields are parallel to the z-axis of LiNbO 3 crystal. We performed FEM simulations of the optical waveguide mode distribution (see Supplementary Fig. 3(a,d)) and the electrostatic field distribution between the metal electrodes corresponding to a set of distances between electrodes(see Supplementary Fig. 3(b,e)) using COMSOL Multiphysics. In particular, we compare a strip waveguide with 2 µm width and 0.6 µm height and a ridge waveguide with 1.5 µm width and 0.35 µm slab thickness. The ridge waveguide shows better performance (lower Vπ values) at the same electrode distance (see Supplementary Fig. 3(c)) due to the presence of LiNbO 3 slab, that "guides" the electric field into the optical mode. In the case of strip waveguide, the modulation  electric field is accumulated mainly in the air gap between the electrodes and the waveguide because of the large dielectric constant (ϵ ≈ 28) of the LiNbO 3 extraordinary crystal axis. On the other side, the tight optical confinement of strip waveguides makes it possible to place electrodes closer to the waveguide, conserving the same added optical loss as for the ridge waveguide. We included optical mode dissipation at the surface of the conductive electrodes to correctly compare the performance of the two platforms based on the numerical calculations of the dielectric function of gold by Werner et al. 4 . As depicted in Fig.3, the strip waveguide has a decrease of less than 20% in voltage length product across the full range of low-loss regimes. At moderate ohmic losses of 0.1 dB/cm, the performance penalty is around 10% and at ohmic losses in excess of 1 dB/cm, the strip waveguide outperforms the ridge waveguide. Here we consider both cases to be air-cladded as was discussed in the main text. The dielectric cladding between electrodes and waveguide can significantly improve electro-optic performance by mitigating the electric field screening effect but might lead to higher optical losses due to the increased evanescent optical field.

Birefringence induced optical mode mixing in waveguide bends
On the ring resonator dispersion measurements, we observe strong mode splitting induced by the mode mixing between different mode families (see Figure 2 main text and Supplementary Figure   4). We identify the mode mixing behaviour as the intermixing of fundamental TE and TM modes by comparison of the group indices of both modes. In contrast to Si 3 N 4 and SI-based optical waveguides, we observe this particular mode mixing behaviour for both microring resonators and racetrack resonators with optimized waveguide bends 5, 6 . We explain this behaviour of strip and ridge waveguides by the phase matching of TE and TM modes due to the material birefringence of the negative uniaxial LiNbO 3 crystal. In any waveguide bend, the optical mode experiences a change of the material index as the projection of its principal axes onto the crystal axes rotates (see Fig. 4(a,b,c)). The in-plane TE mode experiences a much more significant change of material refractive index as its major electric field direction aligns with the extraordinary crystal axes for horizontal waveguides and the ordinary crystal axes for vertical waveguides. Our numerical simulations indicate that the TE and TM modes experience phase matching at specific waveguide direction angles relative to the crystal extraordinary axis (see Fig.4(d,e)). This phase matching induces mode mixing that leads to distortion of the dispersion profile (see Fig. 5(a)). Further simulations reveal that the phase matching of the two modes occurs in any waveguide thicker than 700 nm irrespective of the etching depth, sidewall angles, or slab heights and for both SiO 2 cladded and uncladded waveguides.

Hybrid integrated laser characterization
Supplementary Figure 5 shows the frequency-dependent transmission, cavity linewidth and histogram of intrinsic microresonator loss rate of the LiNbO 3 chips used for hybrid integrated laser demonstration in Figures 3 and 4 of the main manuscript (D101 F2 C4 WG204). We use the funda-mental TE mode in all cases for characterization and laser operation. We employ frequency comb calibrated laser spectroscopy to perform linear characterization of photonic chips 7 .

Mach-Zehnder modulator
We fabricate a MZM modulator with a 1.73 cm length (see Supplementary Fig. 7) in x-cut LiNbO 3 with push-pull configuration that has waveguide width of 1.5 µm and an LiNbO 3 waveguide -electrode gap of 2 µm. LiNbO 3 waveguides are formed by partial etching with a 100 nm slab thickness and the electrodes are fabricated in gold with 500 nm thickness. We use MMI splitters with less than 100 µm length that provide less than 5% imbalance in splitting. Measured V π is 1.94 V (see Supplementary Fig. 7(c)), resulting in V π L=3.3 V×cm.